1. Field of the Invention
This invention relates to methods of and on apparatus for making polymer material and to novel polymer material made by such methods and apparatus.
2. Discussion of the Background
The novel polymer material is useful as a synthetic arterial prosthesis, and the methods and apparatus are adapted to produce such tubular material.
In GB-2 130 521 B there is described the production of a synthetic arterial prosthesis comprising a cellular polyurethane tube preformed by coagulation casting on to a former. A water soluble coating having a smooth surface is separately applied to another former and a polyurethane film is solvent cast on to the coating. The preformed tube is subsequently shrunk on to the film and the former is removed by dissolving the coating in water.
The resultant materials are said to be compliant to pulsatile flow, the inner film being smooth to within a tolerance of 10 microns for minimizing frictional energy losses at the wall of the tube and inhibiting pooling of procoagulents and the adhesion of platelets, which would ultimately give rise to thrombosis. The purpose of the water soluble coating was to produce such a smooth inner surface.
It is now found that it is unnecessary to use such a coating for this purpose.
The shrinking and bonding of the tube on to the film was, in GB-2 130 521 B, effected by heat--the assembly as subjected to a temperature of 100.degree. C. for an hour. The bonded assembly was then suspended in a circulating bath of water typically at 80.degree. C. for half an hour to dissolve the polyvinyl-alcohol release agent.
It is now found that at such high temperatures, extended treatments can degrade the polymer material to the extent that there is a risk of failure in use.